Method and apparatus for cold-drawing metallic tubes

ABSTRACT

Steel tubes are cold-drawn in a bench wherein the reducing die is located in front of a sleeve defining with the external surface of the tube to be cold-drawn an elongated tubular confining space which tapers toward the die at an angle of less than 3*. The space is filled with a lubricant at an elevated pressure of up to 100 atmospheres superatmospheric pressure, and such pressure rises automatically to a value exceeding at least 10 times the initial pressure when the tube is moved lengthwise at a speed of up to and in excess of 300 meters per minute. The rise in pressure of confined lubricant is due in part to friction between the lubricant and the external surface of the tube and suffices to insure that the moving tube remains out of mechanical contact with the die. A similar forwardly tapering confining space for reception of a pressurized lubricant can be provided between the internal surface of the tube and a confining member which is connected to and located behind a floating mandrel. The thus treated tube can be subjected to one or more additional cold-drawing treatments to reduce its outer diameter from 8-35 millimeters to about 3 millimeters and its wall thickness from about 3.5 millimeters to about 1 millimeter.

United States Patent [191 Benteler et al.

[ Mar. 26, 1974 METHOD AND APPARATUS FOR COLD-DRAWING METALLIC TUBES[73] Assignee: Benteler-Werke AG, Schloss Neuhaus, Kreis Padeborn,Germany [22] Filed: June 26,1972

21 Appl. No.: 265,970

[30] Foreign Application Priority Data June 24, 1971 Germany 2131343[52] US. Cl 72/43, 72/60, 72/283 OTHER PUBLICATIONS Promotion of FluidLubrication in Wire Drawing;

by Christopherson et al.; pp. 643-653; Pro. Inst. Mech. Eng, 1955, Vol.169.

Primary Examiner-Richard J. Herbst Attorney, Agent, or Firm-Michael S.Striker [5 7] ABSTRACT Steel tubes are cold-drawn in a bench wherein thereducing die is located in front of a sleeve defining with the externalsurface of the tube to be cold-drawn an elongated tubular confiningspace which tapers toward the die at an angle of less than 3. The spaceis filled with a lubricant at an elevated pressure of up to 100atmospheres superatmospheric pressure, and such pressure risesautomatically to a value exceeding at least 10 times the initialpressure when the tube is moved lengthwise at a speed of up to and inexcess of 300 meters per minute. The rise in pressure of confinedlubricant is due in part to friction between the lubricant and theexternal surface of the tube and suffices to insure that the moving tuberemains out of mechanical contact with the die. A similar forwardlytapering confining space for reception of a pressurized lubricant can beprovided between the internal surface of the tube and a confining memberwhich is connected to and located behind a floating mandrel. The thustreated tube can be subjected to one or more additional cold-drawingtreatments to reduce its outer diameter from 8-35 millimeters to about 3millimeters and its wall thickness from about 3.5 millimeters to about 1millimeter.

25 Claims, 2 Drawing Figures SHEU 1 DP 2 i ATENTED "AR 26 I974 FIG. 2

METHOD AND APPARATUS FOR COLD-DRAWING METALLIC TUBES BACKGROUND OF THEINVENTION The present invention relates to a method and apparatus forcold-drawing metallic tubes or billets, especially steel tubes. Moreparticularly, the invention relates to improvements in a method andapparatus for cold-drawing tubes or billets (hereinafter called tubes)in a draw bench wherein the surfaces of the tubes are separated frominternal and/or external diameterand- /or wall thickness-reducingdevices by layers or films of a suitable lubricant.

When a metallic tube is transported through the reducing die of a drawbench at an elevated speed exceeding 100 meters per minute, properlubrication of the tube andlor of the die presents serious problems.Even a short-lasting interruption in continuity of the film of lubricantcan result in breakage of the moving tube and/or serious damage to thedraw bench, particularly to transporting tools with attendant lengthyinterruptions in operation and losses in output.

It was already proposed to feed a lubricant between a moving elongatedmetallic workpiece and the diameter-reducing device of a draw bench atan elevated pressure which suffices to insure that the external surfaceof the moving workpiece is separated from the reducing device by acontinuous layer of libricant. Wire drawing machines are equipped withapparatus which supply lubricant at a pressure high enough to preventany mechanical contact between the rapidly moving blank and thecomponent parts of the machine. Thus, the reduction of wire diameter insuch machines is attributable mainly to hydrostatic pressure and onlyindirectly to action of the reducing die. The necessary pressurizationof lubricant is brought about artifically by resorting to suitable pumpsor the like.

Attempts to rely on the same principle in the colddrawing of metallictubes have met with little success. The generation of requisitelubricant pressuse by means of pumps presents serious problems andcontributes significantly to the cost of the cold-drawing operation.Moreover, the admission of highly pressurized lubricant into the regionimmediately upstream of the reducing die in a draw bench presentsadditional serious problems. It was further believed that such mode ofcold-drawing cannot be resorted to for simultaneous reduction ofmetallic tubes by a die and by a mandrel which is installed in theinterior of the tube in the region of the die. Heretofore known attemptsto introduce a highly pressurized lubricant between the external surfaceof a mandrel and the internal surface of a long metallic tube have metwith failure due to leakage of pressurized fluid on its way from aremote pressurizing device toward the mandrel. In addition, thepresently known draw benches which operate on the principle ofhydrostatic reduction of the diameter and/or wall thickness of ametallic tube are extremely complex and prone to malfunction so thatthey failed to gain widespread acceptance in the industry.

SUMMARY OF THE INVENTION An object of the invention is to provide anovel and improved method of cold-drawing metallic tubes, particularlysteel tubes, by relying on the pressure of lubricant which coats theinternal and/or external surfaces of tubular workpieces during transportthrough the reducing station.

Another object of the invention is to provide a novel and improvedmethod of insuring the formation of and of maintaining a continuous filmof lubricant between a rapidly moving metallic tube and internal and/orexternal diameterand/or wall thickness-reducing tools in a draw bench.

A further object of the invention is to provide a method which insuressatisfactory cold-drawing of metallic tubes at speeds of up to and inexcess of 300 meters per minute.

An additional object of the invention is to provide a novel and improveddraw bench for cold-drawing of metallic tubes at a high rate of speedand with greatly reduced likelihood of damage to tubes and/or componentparts of the draw bench.

Still another object of the invention is to provide a draw bench withnovel and improved means for supplying and confining pressurizedlubricant at the internal and/or external surfaces of moving metallictubes.

A further object of the invention is to provide a draw bench wherein thepressurization of lubricant takes place automatically when the drawbench is in use and wherein such pressurization invariably suffices toinsure the formation of an uninterrupted film of lubricant betweeninternal and/or external reducing tools and the corresponding surfacesof metallic tubes.

The method of the present invention is utilized for cold-drawing ofmetallic tubes, especially steel tubes, and comprises the steps ofestablishing along at least one of the internal and external surfaces ofthe leading end of a metallic tube a confined tubular layer ofpressurized lubricant which tapers slightly toward a circumferentiallycomplete thickness-' and/or diameterreducing surface of a die or mandrelin a draw bench, and conveying the tube lengthwise relative to thereducing surface at an elevated speed with attendant additionalpressurization of confined lubricant due in part to frictionalengagement between the one surface of the moving tube and the layer ofconfined lubricant to thereby raise the pressure of confined lubricantto a value at which the one surface of the moving tube remains separatedfrom the reducing surface by a continuous film of lubricant. The initialpressure of lubricant is preferably less than atmospheressuperatmospheric pr ess ure, and such pressure is increased to at leastten times the initial pressure when the tube is in motion. The workingpressure of lubricant may be in excess of 600 and preferably in excessof 1,000 atmospheres superatmospheric pressure. The tube can betransported lengthwise at a speed of at least 200 and preferably at orin excess of 300 meters per minute.

The improved cold-drawing method can be practiced with seamless tubes orwith welded tubes, and the diameter of the one surface of the tube canbe reduced from 8-35 millimeters to as low as 3 millimeters. At the sametime, the wall thickness of the moving tube can be reduced from 3.5millimeters or more to less than 1 millimeter.

If desired or necessary, the reduction of the wall thickness and/ordiameter of the tube can be effected in two or more passes whereby thetube is being pulled through a first die by transporting mechanism ormechanisms located between the first and second, second and third, etc.,dies.

The novel features which are considered as characteristic of theinvention are set forth in particular in the appended claims. Theimproved draw bench itself, however, both as to its construction and itsmode of operation, together with additional features and advan tagesthereof, will be best understood upon perusal of the following detaileddescription of certain specific embodiments with reference to theaccompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a fragmentary longitudinalsectional view of a draw bench which embodies the invention; and

FIG. 2 is a diagrammatic plan view of a battery of draw benches andtransporting devices for metallic tubes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to FIG. 1,there is shown a draw bench 2 which is utilized to reduce the internaland external diameters as well as the wall thickness of successivemetallic tubes 1. The draw bench 2 includes a frame having a holder 3for a reducing die 4 which is fixedly but removably received in theholder. The holder 3 further surrounds the front end portion of aconfining sleeve 5 which defines with the leading end of a freshlyintroduced tube 1 a tubular space 12 tapering forwardly toward the die 4at an angle y of not more than 3 and preferably about L5. The space 12surrounds the cylindrical external surface of the leading end of thetube 1 and its length is between 5 and 15 times the external diameter ofthe tube 1 prior to passage through the die 4. The flange 50 at theforward end of the confining sleeve 5 is pressed against the die by anut '7 which meshes with the rear end portion of the holder 3. A sealingring 6 is interposed between the die 4 and flange 5a to prevent escapeof lubricant at the front end of the tubular space 12.

The frame of the draw bench 2 further comprises a ring-shaped housing 8which surrounds the rear end portion of the confining sleeve 5 and isflanked by annular sealing elements 9. A split ring 10 biases thelefthand sealing element 9 against the adjacent end face of the housing8 with a force which suffices to prevent any appreciable escape oflubricant at both axial ends of the housing. The housing 8 can beslipped off the lefthand end of the sleeve 5 upon removal of the splitring 10.

The housing 8 is provided with an internal annular groove or chamber 8awhich communicates with several radial ports 11 of the confining sleeve5; these ports communicate with the rear portion of the tubular space12. An inlet opening 13 of the housing 8 communicates with the chamber8a and is connected with the liquid discharging opening of a pump Pwhich can draw lubricant from a source T. The pump P can be replaced byor provided in addition to an accumulator A. This pump can fill thespace 12 with lubricant at a pressure of between 20-100 atmospheresuperatmospheric pressure.

The draw bench 2 further comprises a floating mandrel 14a which islocated at the front end of an elongated confining member 14 definingwith the cylindrical internal surface of the leading end of the tube 1 asecond tubular confining space 15 which tapers toward the die 4 at anangle 7 of not more than 3 and preferably about 1.5". The axial lengthof the confining space 15 is preferably at least 5 times and up to 15times the initial internal diameter of the tube 1. It will be seen that,in the illustrated embodiment, the axial length of the internalconfining member 14 equals or aproximates the axial length of thesleeve-like external confining member 5.

The cylindrical portion 140' of the floating mandrel 14a is located infront of a portion 14b having a conical reducing surface which divergestoward the space 15 at an angle B of between 5 and 15, preferably about9. The conical portion 4a of the internal surface of the die 4 divergestoward the space 12 at an angle 0: of between 10 and 20, preferablyabout 15. The conical surface of the portion 14b of the mandrel 14a isspaced from and is surrounded by the conical portion 4a of the internalsurface of the die 4.

The reference character 16 denotes a supply pipe which furnishespressurized lubricant from the pump P and/or accumulator A to the rearend of the confining space 15. The initial pressure oflubricant in theconfining space 15 is preferably between 20 and atmospheressuperatmospheric pressure.

A conduit 17 supplies pressurized lubricant to an axial bore 18 whichextends through the confining member 14 and mandrel 14a to admitlubricant to the tubular space 15 of the next draw bench (not shown inFIG. 1) which subjects the once-treated tube 1 to a second cold-drawingtreatment.

FIG. 2 illustrates a combination of several draw benches including thosenumbered 2, 2a, 2b with transporting or pulling devices 30, 30a, 30blocated downstream of the corresponding draw benches. Each pullingdevice comprises a wheel 31 around which the tube 1 forms at least oneconvolution and which can be driven by a variable-speed electric motor32. The exact construction of the pulling devices 30, 30a, 30b isdisclosed in our copending application Ser. No. 265,969 filed June 26,1972 and entitled Multi-pass method and apparatus for cold-drawing ofmetallic tubes.

The lubricant which is furnished by the conduit 17 of FIG. 1 and passesthrough the bore 18 of the confining member 14 and mandrel 14a flowsthrough that portion of the once-treated tube 1 which extends betweenthe draw benches 2, 2a and enters the space 15 in the second draw bench2a. The draw bench 2b may but need not have a mandrel 14a. The conduit17 may be flexible, either entirely or in part, to insure a satisfactoryfeed of pressurized lubrucant in spite of the fact that the mandrel 14afloats in the interior of the tube 1. This conduit 17 can be omitted ifthe tube 1 is to be subjected to a single cold-drawing treatment.

When the draw bench 2 of FIG. 1 is to be put to use, the leading end ofthe tube 1 is introduced between the die 4 and mandrel 14a and isconvoluted onto the wheel 31 of the pulling device 30. The pump P isthereupon started to draw lubricant from the source T, to pressurizesuch lubricant, and to introduce it into the confining spaces 12 and 15at a pressure of between 20 and 100 atmospheres superatmosphericpressure. The motor 32 is started to move the tube 1 lengthwise at aspeed of at least 200 and preferably at least 300 meters per minutewhereby the pressure of lubricant which is confined in the spaces 12 andis automatically rises to at least 600 and preferably LOGO-3,000atmospheres superatmopheric pressure. Such rise in pressure of lubricantis attributed to friction between the external and internal surfaces ofthe tube 1 and the layers of pressurized lubricant in the respectivespaces 12 and 15. The highly pressurized lubricant forms uninterruptedfilms which extend between the external surface of the tube 1 and theinternal surface of the die 4 on the one hand, and between the internalsurface of the tube 1 and the mandrel 14a on the other hand. The slighttaper of spaces 12 and 15, coupled with the high speed of forwardmovement of the tube 1 and initial pressure oflubricant in the spaces 12and 15, insures that the reduction in diameter of the tube 1 and thereduction of its wall thickness can be brought about exclusively byhydrodynamic pressure of the lubricant, i.e., without any mechanicalcontact between the mandrel 14a and/or die 4 and the respective surfacesof the tube 1. The diameter of the tube 1 can be reduced from between 8-35 millimeters to less than 3 millimeters, and the wall thickness of thetube can be reduced from more than 3.5 millimeters to 1.5 millimeters oreven less than i millimeter. As mentioned above, such reduction indiameter and/or wall thickness can be achieved in a single pass or intwo or more passes.

An important advantage of the improved method and draw bench is that thepump P must supply lubricant only prior to and/or during start of acold-drawing operation and only at a pressure which suffices to insuresatisfactory lubrication of the internal and/or external surface of thetube 1. When the operation is started, the pressure of confinedlubricant rises automatically to that value which is necessary to insurecontactless cold-drawing. Thus, the outlay for the means which subectsthe lubricant in the confining space 15 and/or 12 to an initial pressureis only a small fraction of the outlay which would be necessary toartificially maintain the pressure at a value which is needed to insurecontactless cold-drawing while the tube is moved lengthwise at thenormal speed. The hydrodynamic rise in pressure of lubricant as a resultof friction between the moving tube and the stagnant layer or layers ofconfined lubricant is sufficient to insure the formation of apractically uninterrupted film of lubricant between the reducing surfaceor surfaces and the respective surface or surfaces of the tube, even ifthe lubricant is subjected to an initial (artificial) pressure as low asatmospheres superatmospheric pressure.

Another important advantage of the improved method and draw bench isthat a continuous film of lubricant can be established and maintainednot only between a die and the external surface of a tube but alsobetween the internal surface of the tube and a floating mandrel. Thelubricant is sucked and/or pushed into the minute clearances between thereducing surfaces and the corresponding surfaces of the rapidly movingtube wereby the afordescribed length and taper of the spaces 12 and 15contribute to a rapid and substantial rise in the pressure of confinedlubricant to at least ten times the initial pressure. The extent towhich the pressure of confined lubricant will rise also depends on thespeed of lengthwise movement and surface finish of the tube; thesefactors influence the friction between the layers of lubricant and theworkpiece. The actual reduction in the wall thickness and diameter ofthe tube is effected by the highly pressurized lubricant in the regionof conical surfaces on the mandrel 14a and in the die 4. When the drawbench is in use, there develops an equilibrium between the necessarydiameterand wall thickness-reducing pressure and the maximum achievablepressure of lubricant; therefore, the reducing action is practicallyindependent of the initial wall thickness and diameter as well asquality of the material of the tube, as long as the length of the spaces12, 15, their taper toward the die and the finish of surfaces on thetube remain unchanged and the tube is transported at a sufficiently highspeed (a feature which is evidently desirable in cold-drawing ofmetallic tubes).

The taper of the spaces 12 and 15 will be selected in dependency on theroughness or smoothness of the corresponding surfaces of the tube aswell as on the speed at which the tube is transported through the die 4.As mentioned above, the moving tube can increase the initial pressure oflubricant to at least 600 and preferably at least 1,000 atmospheressuperatmospheric pressure, and the speed of the tube 1 is preferably atleast 200 and most preferably about 300 meters per minute, i.e., a speedwhich is much higher than in presently known draw benches.

The improved method and apparatus can be utilized for treatment ofseamless metallic tubes as well as for treatment of welded tubes. Theinitial diameter of the tubes is preferably in the range of 8-35millimeters and the initial wall thickness is less than 3.5 millimetersor even less than 1.5 millimeters. As mentioned above, the wallthickness of the tubes can be reduced to less than 1 millimeter andtheir diameter to less than 3 millimeters, especially by utilizing abattery of two or more draw benches and interposed transporting orpulling devices.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can, by applying current knowledge,readily adapt it for various applications without omitting featureswhich fairly constitute essential characteristics of the generic andspecific aspects of our contribution to the art and, therefore, suchadaptations should and are intended to be comprehended within themeaning and range of equivalence of the claims.

We claim:

1. A method of cold-drawing metallic tubes, particularly steel tubes,which comprises establishing along the internal and external surfaces ofthe leading end of a tube confined layers of pressurized lubricant whichrespectively taper slightly forwardly toward a circumferentiallycomplete thickness-reducing external surface of a floating mandrel andtoward a circumferentially complete thickness-reducing internal surfaceof a reducing die; and conveying the tube lengthwise relative to thereducing surfaces at an elevated speed with attendant additionalsubstantial pressurization of lubricant due to frictional engagementbetween the layers of lubricant and said internal and external surfacesto thereby raise the pressure of confined lubricant to a value at whichsaid internal and external surfaces of the moving tube remain separatedfrom the respective reducing surfaces by continuous films of lubricant.

2. A method as defined in claim 1, wherein the length of one of saidconfined layers, as considered in the direction of movement of the tube,equals or approximates the length of the other of said layers.

3. A method as defined in claim 1, wherein the initial pressure oflubricant forming said layers is less than atmospheres superatmosphericpressure and said value is at least ten times said initial pressure.

4. A method as defined in claim 3, wherein said value is at least 600atmospheres superatmospheric pressure.

5. A method as defined in claim 1, wherein said conveying step comprisesmoving the tube at a speed of at least 200 meters per minute.

6. A method as defined in claim wherein said speed is in excess of 300meters per minute.

7. A method as defined in claim 1, wherein said tube is a seamless tube.

8. A method as defined in claim 1, wherein said tube is a welded tube.

9. A method as defined in claim 1, wherein the initial diameter of saidexternal surface of the tube is between 8 and 35 millimeters;

10. A method as defined in claim 1, wherein the initial wall thicknessof the tube is less than 3.5 millimeters.

11. A method as defined in claim 10, wherein the initial wall thicknessof the tube is less than 1.5 millimeters.

12. A method as defined in claim I, wherein said conveying stepcomprises pulling the tube at at least one point located downstream ofsaid thickness-reducing surfaces, and further comprising the step ofsubjecting the once drawn tube to at least one additional cold drawingoperation with attendant additional reduction in wall thickness andexternal diameter of the tube.

13. A method as defined in claim 12, wherein the final wall thickness ofthe tube at most equals or slightly exceeds 1 millimeter.

14. A method as defined in claim 12, wherein the final outer diameter ofthe tube approximates 3 millimeters.

15. In a draw bench for cold-drawing of metallic tubes, particularlysteel tubes, a combination comprising a reducing die; first confiningmeans located upstream of said die and defining with the externalsurface of a tube which extends into said die a dirst tubular spacetapering toward said die; a mandrel disposed within the tube in theinterior of said die; second confining means dieposed behind saidmandrel and defin ing with the internal surface of the tube a secondtubular space which tapers toward said mandrel; means for introducinginto said tubular spaces a lubricant at an elevated pressure so thatsuch lubricant completely fills said tubular spaces; and conveying meansfor moving the tube lengthwise at an elevated speed with attendantadditional substantial pressurization of confined lubricant in saidtubular spaces due to frictional engagement between the lubricant andthe external and internal surfaces of the moving tube to thereby raisethe pressure of lubricant in said tubular spaces to a value at whichsaid external and internal surfaces of the moving tube respectivelyremain separated from said die and from said mandrel by continuous filmsof lubricant.

16. A combination as defined in claim 15, wherein the taper of saidspaces is in the range of at most about 17. A combination as defined inclaim 16, wherein the taper of said spaces is at most l.5.

18. A combination as defined in claim 15, wherein the axial length ofsaid first space is between 5 and 15 times its internal diameter.

19. A combination as defined in claim 15, wherein said die has aninternal surface including a conical portion diverging toward said firstspace at an angle of between l0 and 20.

20. A combination as defined in claim 15, wherein the taper of saidsecond space is in the range of at most 3.

21. A combination as defined in claim 20, wherein the taper of saidsecond space is at most l.5.

22. A combination as defined in claim 15, wherein the axial length ofsaid second space is between 5 and 15 times its external diameter.

23. A combination as defined in claim 15, wherein said mandrel has anexternal surface including a conical portion which diverges toward saidsecond space at an angle of between 5 and l5.

24. A combination as defined in claim 15, wherein the length of saidfirst tubular space equals or approximates the length of said secondtubular space.

25. In an apparatus for cold-drawing of metallic tubes, particularlysteel tubes, a combination comprising a battery of at least two drawbenches each having a reducing die, first confining means locatedupstream of the die and defining with the external surface of a tubewhich extends into said die a tubular space tapering toward the die, afloating mandrel disposed within the tube in the interior of the die,second confining means disposed behind said mandrel and defining withthe internal surface of the tube a second tubular space which taperstoward said mandrel, and means for introducing into said tubular spacesa lubricant at an elevated pressure so that the lubricant completelyfills said spaces; first conveying means located downstream of that oneof said draw benches through which a tube passes prior to entering theother of said draw benches; and second conveying means locateddownstream of said other draw bench, said first and second conveyingmeans being arranged to move a tube lengthwise at an elevated speed withattendant additional substantial pressurization of confined lubricant insaid spaces of said draw benches due to frictional engagement betweenthe lubricant and the external and internal surfaces of the moving tubeto thereby raise the pressure of lubricant in said spaces to a value atwhich said external and internal surfaces of the moving tuberespectively remain spaced from said dies and from said mandrels bycontinuous films of lubricant.

1. A method of cold-drawing metallic tubes, particularly steel tubes,which comprises establishing along the internal and external surfaces ofthe leading end of a tube confined layers of pressurized lubricant whichrespectively taper slightly forwardly toward a circumferentiallycomplete thickness-reducing external surface of a floating mandrel andtoward a circumferentially complete thickness-reducing internal surfaceof a reducing die; and conveying the tube lengthwise relative to thereducing surfaces at an elevated speed with attendant additionalsubstantial pressurization of lubricant due to frictional engagementbetween the layers of lubricant and said internal and external surfacesto thereby raise the pressure of confined lubricant to a value at whichsaid internal and external surfaces of the moving tube remain separatedfrom the respective reducing surfaces by continuous films of lubricant.2. A method as defined in claim 1, wherein the length of one of saidconfined layers, as considered in the direction of movement of the tube,equals or approximates the length of the other of said layers.
 3. Amethod as defined in claim 1, wherein the initial pressure of lubricantforming said layers is less than 100 atmospheres superatmosphericpressure and said value is at least ten times said initial pressure. 4.A method as defined in claim 3, wherein said value is at least 600atmospheres superatmospheric pressure.
 5. A method as defined in claim1, wherein said conveying step comprises moving the tube at a speed ofat least 200 meters per minute.
 6. A method as defined in claim 5wherein said speed is in excess of 300 meters per minute.
 7. A method asdefined in claim 1, wherein said tube is a seamless tube.
 8. A method asdefined in claim 1, wherein said tube is a welded tube.
 9. A method asdefined in claim 1, wherein the initial diameter of said externalsurface of the tube is between 8 and 35 millimeters.
 10. A method asdefined in claim 1, wherein the initial wall thickness of the tube isless than 3.5 millimeters.
 11. A method as defined in claim 10, whereinthe initial wall thickness of the tube is less than 1.5 millimeters. 12.A method as defined in claim 1, wherein said conveying step comprisespulling the tube at at least one point located downstream of saidthickness-reducing surfaces, and further comprising the step ofsubjecting the once drawn tube to at least one additional cold-drawingoperation with attendant additional reduction in wall thickness andexternal diameter of the tube.
 13. A method as defined in claim 12,wherein the final wall thickness of the tube at most equals or slightlyexceeds 1 millimeter.
 14. A method as defined in claim 12, wherein thefinal outer diameter of the tube approximates 3 millimeters.
 15. In adraw bench for cold-drawing of metallic tubes, particularly steel tubes,a combination comprising a reducing die; first confining means locatedupstream of said die and defining with the external surface of a tubewhich extends into said die a dirst tubular space tapering toward saiddie; a mandrel disposed within the tube in the interior of said die;second confining means dieposed behind said mandrel and defining withthe internal surface of the tube a second tubular space which taperstoward said mandrel; means for introducing into said tubular spaces alubricant at an elevated pressure so that such lubricant completelyfills said tubular spaces; and conveying means for moving the tubelengthwise at an elevated speed with attendant additional substantialpressurization of confined lubricant in said tubular spaces due tofrictional engagement between the lubricant and the external andinternal surfaces of the moving tube to thereby raise the pressure oflubricant in said tubular spaces to a value at which said external andinternal surfaces of the moving tube respectively remain separated fromsaid die and from said mandrel by continuous films of lubricant.
 16. Acombination as defined in claim 15, wherein the taper of said spaces isin the range of at most about 3*.
 17. A combination as defined in claim16, wherein the taper of said spaces is at most 1.5*.
 18. A combinationas defined in claim 15, wherein the axial length of said first space isbetween 5 and 15 times its internal diameter.
 19. A combination asdefined in claim 15, wherein said die has an internal surface includinga conical portion diverging toward said first space at an angle ofbetween 10 and 20*.
 20. A combination as defined in claim 15, whereinthe taper of said second space is in the range of at most 3*.
 21. Acombination as defined in claim 20, wherein the taper of said secondspace is at most 1.5*.
 22. A combination as defined in claim 15, whereinthe axial length of said second space is between 5 and 15 times itsexternal diameter.
 23. A combination as defined in claim 15, whereinsaid mandrel has an external surface including a conical portion whichdiverges toward said second space at an angle of between 5 and 15*. 24.A combination as defined in claim 15, wherein the length of said firsttubular space equals or approximates the length of said second tubularspace.
 25. In an apparatus for cold-drawing of metallic tubes,particularly steel tubes, a combination comprising a battery of at leasttwo draw benches each having a reducing die, first confining meanslocated upstream of the die and defining with the external surface of atube which extends into said die a tubular space tapering toward thedie, a floating mandrel disposed within the tube in the interior of thedie, second confining means disposed behind said mandrel and definingwith the internal surface of the tube a second tubular space whichtapers toward said mandrel, and means for introducing into said tubularspaces a lubricant at an elevated pressure so that the lubricantcompletely fills said spaces; first conveying means located downstreamof that one of said draw benches through which a tube passes prior toentering the other of said draw benches; and second conveying meanslocated downstream of said other draw bench, said first and secondconveying means being arranged to move a tube leNgthwise at an elevatedspeed with attendant additional substantial pressurization of confinedlubricant in said spaces of said draw benches due to frictionalengagement between the lubricant and the external and internal surfacesof the moving tube to thereby raise the pressure of lubricant in saidspaces to a value at which said external and internal surfaces of themoving tube respectively remain spaced from said dies and from saidmandrels by continuous films of lubricant.